The present invention relates to a process for the removal of organic impurities in a Bayer process liquor.
The accumulation of organic impurities in Bayer process liquors is a problem faced by most alumina refineries. The bulk of these impurities enter as contaminants within the bauxite, although a small proportion enters the liquor stream as a result of process additives such as flocculants and antifoams. Because Bayer liquors are highly caustic most of the organic compounds hydrolyse and are therefore present as their sodium salts. Caustic-insoluble organics generally depart with the mud residue and play no further part in the Bayer process. Apart from the direct deleterious effects of these organic species on the alumina refinery""s productivity and product quality, a proportion decomposes to form sodium carbonate and sodium oxalate. These latter contaminants create a variety of problems in their own right, including caustic consumption, reduced yield and degraded product quality. Consequently, most refineries already operate processes to control the levels of sodium carbonate (by causticisation with slaked lime) and oxalate. However, far fewer refineries practice organic impurity removal, and the major reason for this is that existing organic removal processes are either complicated, expensive, or form side-products that are almost as problematical as the organics themselves.
Most organic removal processes operate via some variation on the principal of oxidative destruction of the organics. These processes can be performed either in the solid phase (calcination or liquor burning technologies) or in the aqueous phase (xe2x80x9cwet oxidationxe2x80x9d technology, in which the oxidation is effected either by chemical or electrical means). Both the xe2x80x9csolidxe2x80x9d (dry) and xe2x80x9cliquidxe2x80x9d (wet) phase processes suffer some serious disadvantages, which will be discussed below. Other organic removal processes such as the use of liquid anion exchange resins, ultrafiltration, or adsorbent materials such as magnesite, are of no interest in the present application, and will not be considered.
Two of the more commonly used dry organic destruction processes include liquor burning and salting-out evaporation. In the former process, liquor is evaporated to dryness in contact with gibbsite or alumina to form pellets which are then calcined. The oxidation products, along with the sodium carbonate and sodium hydroxide in the liquor react with the alumina to form sodium aluminate, which is subsequently dissolved and returned to the process. Thus, the process xe2x80x9ccausticisesxe2x80x9d the organics, recovering the valuable soda. Unfortunately, the process is complicated and energy-inefficient, most of the energy being consumed in evaporating the water from the feed liquors.
The process of salting-out evaporation is similar. In this case, a liquor stream is deeply evaporated, resulting in the xe2x80x9csalting-outxe2x80x9d of impurities, such as organic sodium salts, sodium oxalate, sodium carbonate and sodium sulphate. The solid impurities are separated from the supernatant liquor by filtration or centrifugation. The filtrate or centrate is returned to the process, while the solids are either disposed directly (resulting in a substantial loss of soda values), reacted with lime to causticise the sodium carbonate component, or mixed with bauxite and fed to a kiln. In the kiln, the carbonate, oxalate and organic species react with the bauxite to form mainly sodium aluminate and sodium ferrate. The kiln products are then reslurried and directed either to the digestion circuit, or the clarification circuit of the Bayer process. The salted-out solids are often very viscous and poorly crystalline, and can be difficult to separate from the supernatant liquor. Like liquor burning, the process is also very energy-inefficient requiring the evaporation of large quantities of water.
Wet oxidation processes involve reaction of the organic species with an oxidising agent. such as oxygen, ozone, chlorine or manganese dioxide. Contamination of the liquor stream, toxicity and reagent costs are prohibitive with most reagents other than oxygen. Oxidation using oxygen or ozone can be effective and economical, but requires operation at elevated temperatures and pressures for maximum efficiency. Safety is a serious concern with this process, as dangerous levels of hydrogen can be evolved in these high temperature processes. Electrolytic processes have been investigated at a laboratory level, but remain untried on a pilot or plant scale.
All of the wet oxidation processes suffer from a serious disadvantage in that they produce large quantities of sodium carbonate, and in most cases, sodium oxalate. This places considerable strain upon the refinery""s existing carbonate and oxalate removal facilities. In practice, this will usually necessitate the construction of additional causticisation and oxalate removal capacity, together with increased consumption of reagents such as lime.
The present invention was developed with a view to providing an improved organic removal process in which wet and dry organic removal processes are combined in a complementary manner such that the weaknesses of each individual process become a strength of the combined process.
According to one aspect of the present invention there is provided a process for the removal of organic impurities from a Bayer process liquor, the process including the steps of:
feeding a Bayer liquor stream rich with organic impurities to a wet oxidation process to produce a first processed liquor which is depleted in organic compounds, but enriched with sodium carbonate and/or sodium oxalate;
reacting a substantial component of the sodium compounds in a feed slurry using a dry oxidation process to produce a processed discharge product,
feeding at least a portion of the first processed liquor to a leach tank liquor to which is added the processed discharge product from the dry oxidation process, wherein the sodium carbonate and/or sodium oxalate precipitate in the leach tank liquor; and,
separating the precipitated sodium carbonate and/or sodium oxalate from the leach tank liquor and recycling the precipitated products in the feed slurry to the dry oxidation process;
whereby, in use, organic impurities in the Bayer liquor stream and residual organic impurities remaining in the first processed liquor or in the recycled precipitated products, are causticised to sodium aluminate or sodium ferrate in the dry oxidation process.
Preferably, substantially all of the Bayer liquor stream is fed to the wet oxidation process first and the balance (if any) of the first processed liquor (that which is not fed to the leach tank liquor), is fed to the dry oxidation process.
Typically said balance of the first processed liquor and the recycled precipitated products are fed to a mix tank for the dry oxidation process.
Typically the dry oxidation process employs a liquor burner. Optionally the wet oxidation process also employs an evaporator. Advantageously the process of the present invention is combined with a sulphate removal process which is the subject of Australian patent No. 673306, the contents of which are incorporated herein by reference.
In such an arrangement, a proportion of the processed discharge product from the dry oxidation process is fed to a second leach tank liquor having a caustic concentration sufficient to ensure the solubility of gibbsite is not exceeded, and wherein the feed of said processed discharge product to the second leach tank liquor is regulated to ensure that the amount of sodium sulphate in said processed discharge product is substantially equal to the total input of sulphate to the process.